Method of making an encapsulated thick film resistor and associated encapsulated conductors for use in an electrical circuit

ABSTRACT

A method of making an encapsulated thick film resistor and an associated encapsulated conductor so that the stability of the resistor will be maintained at a precise level over an abnormally long period of time by first firing selected mixtures of positive and negative thick film resistor ink materials on a nonelectrically conductive substrate at a selected high temperature, jointly firing the resistor and conductor ink materials associated with this resistor at a temperature that is lower than the first mentioned temperature and which is at a level that will not allow any detrimental diffusion to occur between the conductor and the resistor materials, applying a resilient buffer material formed of a silicone polymer filled with magnesium oxide over the fired resistor and the fired conductor and then applying a hard outer glyptal coating over the resilient buffer.

United States Patent [1 1 [111 3,922,388 Schebalin Nov. 25, 1975 [54]METHOD OF MAKING AN ENCAPSULATED 3,669,650 10/1972 Cocca 117/212 IC LRESISTOR AND ASSOCIATED 3,833,407 9/1974 Schebalin 117/212 ENCAPSULATEDCONDUCTORS FOR USE IN AN ELECTRICAL CIRCUIT Inventor:

Nov. 20, 1974 Appl. No.: 525,587

Related U.S. Application Data Continuation of Ser. No. 334,956, March22, 1973,

\ abandoned, which is a division of Ser. No. 120,199,

March 2, 1971, Pat. No. 3,788,891."

U.S. Cl. 427/103; 427/102; 338/308 Int. Cl. I-IOIH 37/36; 11018 l/02Field of Search 117/212, 215, 217, 227,

References Cited UNITED STATES PATENTS Morty et al. 252/514 Dates etal....

Hoffman Youmans 117/212 Primary ExaminerMichael F. Esposito Attorney,Agent, or Firm-Arthur H. Swanson; Lockwood D. Burton; J. Shaw Stevenson[5 7] ABSTRACT A method of' making an encapsulated thick film resistorand an associated encapsulated conductor so that the stability of theresistor will be maintained at a precise level over an abnormally longperiod of time by first firing selected mixtures of positive andnegative thick film resistor ink materials on a non-electricallyconductive substrate at a selected high temperature, jointly firing theresistor and conductor ink materials associated with this resistor at atemperature that is lower than the first mentioned temperature and whichis at a level that will not allow any detrimental diffusion to occurbetween the conductor and the resistor materials, applying a resilientbuffer material formed of a silicone polymer filled with magnesium oxideover the fired resistor and the fired conductor and then applying a hardouter glyptal coating over the resilient buffer.

1 Claim, 9 Drawing Figures "/ePOSITlVE TCR INK U.S. Patent Nov. 25, 1975Sheet10f5 3,922,388

% POSlTlVE TCR INK U.S. Patnt Nov.25, 1975 Sheet20f5 3,922,388

US. Patent Nov. 25, 1975 Sheet40f5 3,922,388

DEPENDENCE OF TCR ON THE ASPECT RATIO (GEOMETRY) OF A RESISTOR 2(IiPM/Cm 00 w m w w m m m m m 5 m w 0022mm 2 mokmamm O moh ASPECTRATIO(GEOMETRY) OF A RESISTOR US. Patent Nov. 25, 1975 RESISTIVITY.QJEI/ MILL Sheet 5 0f 5 FIG. 9

DEPENDENCE OF RESISTIVITY ON ASPECT RATIO (GEOMETRY) OF A RESISTOR Bnooxuumuu.

ZISOIL/El/MILL c IISOJL/EI/MILL ASPECT RATIO METHOD OF MAKING ANENCAPSULATED THICK FILM RESISTOR AND ASSOCIATED ENCAPSULATED CONDUCTORSFOR USE IN AN ELECTRICAL CIRCUIT This application is a continuation ofmy prior application bearing Ser. No. 334,956, filed Mar. 22, 1973,

now abandoned, which is a division of US. patent apl plication Ser. No.120,199, filed Mar. 2, 1971, now US. Pat. No. 3,788,891.

BACKGROUND OF THE INVENTION Field of Invention DESCRIPTION OF THE PRIORART Prior to the present invention, coatings of glass, vinyl paint,acrylic paints, varnishes and different types of epoxy materials havebeen employed as hard coatings to cover thick film resistors. Such hardcoatings are necessary to protect these resistors from scratches andfrom exposure to moisture and undesired corrosive gases in theatmosphere, such as chlorine gas, which have a tendency to corrode theresistor and alter its resistivity.

In the prior art, attempts to select a hard coating to cover thick filmresistors that have exactly the same thermal expansion as the resistorthat it covers so that stresses would not be introduced into theresistor and the coating during a change in ambient temperature have notbeen able to be achieved. Stresses were introduced into the resistor andits associated hard coating and transferred between the resistor andcoating because slight differences were always present between thethermal expansion of the resistor. and the thermal expansion of any oneof the aforementioned hard coatings. Furthermore, as the magnitude ofthe ambient temperature change increases, the magnitude of theaforementioned stresses that are introduced into the resistor andcoating also simultaneously increased. This change in stress of theresistor is also simultaneously accompanied by an undesired change inthe resistance of the thick film resistor.

The value of a resistor must be maintained within plus or minus 0.1% inorder to be classified as a precision thick film resistor. Since none ofthe aforementioned prior art encapsulated thick film resistors can bemaintained within this i 0.1% resistor value, none of these prior arthard coated encapsulated resistors have been found to be satisfactoryfor use as precision thick film resistors.

Another reason why a thick film resistor that is coated by any one ofthe aforementioned coatings is not satisfactory is that when theaforementioned stresses are introduced into these parts, due to theirdifferences in thermal expansion, cracks formed in their coatings andthis allowed the thick film resistor to become exposed to harmfulatmospheric gases, such as previously mentioned chlorine gas,

Heretofore, it was a common practice, after the selection of the sizeand the aspect ratio of a resistor which would fall within a prescribedresistance range, to consult a table or a graph filled geometrycorrecting tables that predicts the resistivity and the T.C.R. as 3 2function of the resistors geometry. This procedure is slow, tedious, andhas circuit design limitations in that the resistor could only be madeof a certain geometric shape and size. It is also well known that theseresistors will have undesired different T.C.R. and resistivity values.

SUMMARY OF THE INVENTION It is a major object of the present inventionto disclose a unique method for making an encapsulated thick filmresistor having encapsulated conductive end portions, which resistor isknown in the art as a cermet resistor.

It is another object of the present invention to provide a method formanufacturing an encapsulated resistor of the aforementioned type thatpossesses electrical resistance characteristics, that are precise andwhose performance is unaffected by the time it remains on the shelf, thetime period over which it is employed in an electrical circuit orchanges in ambient temperature.

More specifically, it is another object of the invention to provide amethod of manufacturing a precision encapsulated resistor of theaforementioned type whose .resistance will remain within an acceptable i0.1% level over long periods of use that extend beyond a two year periodof time.

It is another object of the invention to provide a method ofmanufacturing an encapsulated cermet resistor for use in measuringcircuits whose accuracy and overall stability is as good and reliable asthose possessed by present day commercially available wire woundresistors.

One of the terms that is used to define a critical characteristic of athick film resistor is its sheet resistivity". This term sheetresistivity relates to the electrical resistance which a l milli-inchthick square of any size of resistive material offers to a steadycurrent passing between any two opposite faces of this resistivematerial along which, for example, a conductive film is attached. Thissheet resistivity is known to vary with ambient temperature between,e..g., +300 PPM/"C to 300 PPM/C depending on the sheet resistivity ofresistor material being used.

Another term that is used to define the characteristic of a thick filmresistor is T.C.R. or temperature coefficient of resistivity whicch isthe change in resistivity expressed in ohms per degree centigrade.

In achieving the aforementioned objectives, it has been discovered thatan adverse change in resistivity and T.C.R. of a thick film cermetresistor is caused by diffusion of the conductive material in theconductor, which has an extremely low resistivity value, intoresistive'material of the resistor which has a much greater resistivitywhen the resistor and conductor are fired on a ceramic substrate.Heretofore it was a common belief that this adverse change was basedupon the geometry, or the so-called aspect ratio factor which is a ratioof the length to width of the resistor.

It is another object of the invention to recognize for the first timethat the aforementioned detrimental effect of diffusion is much greaterbetween the ends of a rectangular strip of resistor and a conductor thatextends away from the resistor when the longest-opposite sides of therectangular resistor strip are selected for connection to the conductorfor jointly firing onto a substrate rather than the shorter oppositesides of the rectangular resistor strip.

.Furthermore, experimentation has shown that firing temperature changesadversely affects T.C.R. andthe resulting resistivity of thick filmresistors because of the high degree of the aforementioned diffusionthat takes place between the resistor and the associated conductors towhich it is attached when they are jointly fired.

It is, therefore, another object of the invention to provide auniquemethod of firing resistor and conductor inks onto substrates sothat no undesired diffusion will take place between the resistive andconductive materials that will adversely affect the T.C.R. andresistivity; and, therefore, the precise resistance offered by theresistor.

To accomplish the aforementioned feat it is another object of theinvention to provide a means whereby the dried resistor ink is firstfired for a preferred preselected period of time, e.g., minutes at ahigh temperature of, e.g., 1000C on a substrate to form an amorphic massand thereafter the conductor extending from either side of the resistoris printed, dried and then fired for a similar period of time at asubstantially lower temperature in the neighborhood of 550C, onto thealready firedresistor to eliminate substantially all of the undesireddiffusion of the conductive material that would otherwise diffuse intoor from the resistor material.

It is another object of the invention to provide a method of theaforementioned type which will allow an ink,such as a resistor inkhavinga high firing temperature, to be fired at a high temperature onto asubstrate, and a conductor ink having a lower firing temperature thanthe resistor ink to be then fired jointly with portions of the alreadyfired resistor ink onto the substrate so that undesired diffusion of theconductor ink material into the fired resistor ink will be negligibleand an acceptable cermet resistor having a low T.C.R. to be produced.

It is, also, another object of the invention to eliminate the need forthe previously mentioned geometry correcting tables.

It is also another object to provide a method of manufacturing a cermetresistor whose shape can be of any one of a number of different forms orconfigurations, and need not, therefore, be limited to a restrictedshape as has heretofore been required.

lt-is; another object of the invention to provide a method of blendingone or more positive T.C.R. resistor inks with one or more negativeT.C.R. resistor inks so that the resulting temperature coefficient ofresistivity T.C.R. and the resistivity of the resulting resistor can beprecisely predicted by changing the blending proportions of the negativeT.C.R. resistor ink and the positive T.C.R. resistor ink before firingin the aforementioned unique manner so that a number of different shapedresistors can be formed which individually possess different preciselyfixed resistance values.

It is another object to provide resistors of the aforementioned typethat extend over a wide range and which will result in each of theresistors having a temperature coefficient of resistivity value, T.C.R.,that is within a few parts per million per degrees centigrade from zero.

Since it is not possible to obtain a precise resistance value for theresistor from the aforementioned unique firing process nor from anyother firing process, it is, therefore, another object of the presentinvention to provide a means of trimming such a resistor after ithas yin amorphic form at l0O0C into a sintered state onto 4 an electricallynon-conductive substrate, the third step been fired so that a more exactvalue of the resistance can be achieved for these resistors.

It is another object of the invention to provide a method of making anencapsulated cermet resistor and associated encapsulated conductor sothat no undesired stresses will be introduced into the resistor and theresistance of the resistor to be altered thereby.

More specifically, it is a major object of the present invention toprovide a method of the aforementioned type in which the resistor andassociated conductors are first coated with a resilient buffer material,formed of silicone polymer filled with magnesium oxide, and then coatedwith a hard outer glyptal coating.

It is another object of the present invention to provide a method of theaforementioned type wherein the buffer coating is employed to eliminateany thermally induced stresses in the resistor and hard outer coating asthermal expansion of the resistor and thermal expansion of the hardouter coating takes place, thereby eliminating the possibility of cracksin the hard outer glyptal coating.

It is still another object of the present invention to employ theaforementioned encapsulating method as a way of preventing theresistivity and the T.C.R. value of the resistor produced by this methodfrom being changed by exposure to the destructive oxidation of air,moisture, hydrogen sulfide or other similar ambient atmospheres.

It is another object of the invention to provide a method of blendingone or more positive T.C.R. resistor inks with one or more negativeT.C.R. resistor inks so that the resulting temperature coefficient ofresistivity, T.C.R., and the resistivity of the resulting resistor canbe precisely predicted by changing the blending proportions of thenegative T.C.R. resistor ink and the positive T.C.R. resistor ink beforefiring in the aforementioned unique manner so that a number of differentshaped resistors can be formed which individually possess differentprecisely fixed resistance values.

I It is another object to provide encapsulated resistors of theaforementioned type that extend over a wide range and which will resultin each of the resistors having a temperature coefficient of resistivityvalue, T.C.R., that is within a few parts per million per degreescentigrade from zero.

Since it is not possible to obtain a precise resistance value for theresistor from the aforementioned unique firing process nor from anyother firing process, it is, therefore, another object of the presentinvention to provide a means of trimming such a resistor after it hasbeen fired so that a more exact value of the resistance can be achievedfor these resistors,

ln accomplishing these and other objects, there has been provided inaccordance with the present invention a method of making an encapsulatedthick film resistorhaving conductive portions associated with oppositeends thereof that form portions of an electrically conductive circuit,comprising the first step of selecting an ink made from a mixture ofpositive and negative resistor inks, the second step of firing theresistor ink mix of selecting a conductive ink material that has asintering temperature substantially lower than the sintering temperatureof the resistor, the fourth step of firing said conductive ink at asintering temperature of about 550C in overlapping relationship withsaid opposite ends of said resistor on the substrate whereby thetendency to diffusion between the conductive and resistive materials isminimized and whereby a resulting temperature coefficient of resistivityvalue of the resistor that is produced is within plus or minus twentyparts per million per degree centigrade from zero, the fifth step ofapplying a coating of resilient buffer material formed of a siliconepolymer filled with magnesium oxide to cover said resistor and the saidfired conductive ink and the sixth step of applying a layer of ironoxide with magnesium silicate suspended in xylene to cover the outersurface of said resilient material.

BRIEF DESCRIPTION OF THE DRAWING A better understanding of the presentinvention may be had from the following detailed description when readin connection with the accompanying drawings in which:

FIG. 1 shows a nomograph having a uniquely constructed semi-log scalefor graphically determining the amount of additional positive ornegative ink that should be added to an ink mixture of positive andnegative inks to provide a thick film resistor ink of a desired zeroT.C.R.;

FIG. 2 shows the steps required in a first method of trimming theaforementioned thick film resistor;

FIG. 3 shows the first step required in a second method of trimming theaforementioned thick film resistor;

FIG. 4 shows the second step required in the second method of trimming athick film resistor;

FIG. 5 shows the third step required in the second method of trimmingthe aforementioned thick film resistor;

FIG. 6 shows how the aforementioned trimmed thick film resistor can beencapsulated to prevent the de structive oxidating effect of the ambientatmosphere from affecting its resistivity and T.C.R. value; and

FIG. 7 shows a chart having a solid line thereon to indicate the wideresistivity range of values over which a zero T.C.R. prevails for manydifferent positive and negative cermet resistor ink blends when they areproduced by the unique method to be hereinafter described in which nodiffusion is allowed to occur be tween the conductor and the resistor ascontrasted by the line shown in dash line thereon which indicates thatzero T.C.R. can be achieved for only a single resistivity value of manypsoitive and negative cermet resistor inks when they are produced by thewell known profile firing method as a result of undesired diffusionoccurring between the conductor and its associated resistor.

FIG. 8 is a graph to vividly illustrate the desirable independentrelationship that can be achieved, as shown by curve B, between thetemperature coefficient of resistivity and the aspect ratio (geometry)values of thick film resistors by firing them in the previously referredto unique non-diffused manner with their associated conductors onto asubstrate. FIG. 8 also shows a curve A which represents the undesireddependent, restricted, temperature coefficient of resistivity versusaspect ratio (geometry) relationship that must be adhered to when thickfilm resistors and their associated conductors are fired jointly at ahigh temperature which causes diffusion to occur between the lastmentioned conductors and their associated resistors.

FIG. 9 is a graph to vividly illustrate the desirable independentrelationship that can be achieved as shown by curve B between theresistivity and the aspect ratio (geometry) values of thick filmresistors by firing them in the previously referred to uniquenon-diffused manner with their associated conductors onto a substrate.FIG. 9 also shows a curve A which represents the undesirable dependentrestricted resistivity versus aspect ratio (geometry) relationship thatmust be adhered to when thick film resistors and the associatedconductors are fired jointly at a high temperature which causesdiffusion to occur between the last mentioned conductors and theirassociated resistors.

DESCRIPTION OF THE PREFERRED EMBODIMENT Method of Blending Cermet Inksto Fabricate Encapsulated Thick Film Resistors Having Encapsulated EndPortions Which Are Not Sensitive to Temperature Charges The temperaturecoefficient of resistivity, T.C.R., for thick film cermet resistors hasheretofore been changed by altering the firing temperature profileand/or by changing the geometry, or in other words, the previouslyreferred to aspect ratio of these resistors.

Since the changes in T.C.R. obtained by these methods are several partsper million per degree centigrade,

PPM/C, usually in the vicinity of l to 10 PPM/C for 1 C change in firingtemperature, they are, therefore, not sufficiently exact to obtain thedesired T.C.R. value.

A unique method of ink blending to obtain a desired T.C.R. value whichdoes not have to rely on the selection of a desired firing temperatureprofile will now be described.

The magnitude of change of T.C.R. obtained by this unique method is atleast 10 times larger than the previously mentioned method which wasbased upon a change in firing profile and a change in the geometry ofthe resistor.

Experimentation has shown that an addition of a metal in powder formsuch as a gold powder with the particle size of three to twenty micronsor a metal powder mixed with lead-boro-silicate glass powder of the sameparticle size when mixed with a liquid agent such as decanol provide asuspension that will decrease the sheet resisitivity of a resistor andcause a change in its T.C.R. in a positive direction. The addition ofmetal oxide powder, for example, ruthenium oxide powder,

or a metal oxide powder mixed with boro-lead-silicate glass powder and aliquid agent such as decanol causes a change in T.C.R. in a negativedirection. It can, therefore, be concluded that by adding metal or metaloxide to a cermet ink the T.C.R. is changed in either a desired positiveor a negative direction; and, therefore, if two or more resistor inksare available and if one of them has a positive T.C.R. and the other anegative T.C.R. or vice versa, they may be blended to obtain a desiredT.C.R., and the blending proportion can be calculated by the method tobe hereinafter described:

Measure the T.C.R. of resistors made from the ink which is to bemodified to obtain a zero T.C.R. resistor. This measurement of T.C.R. isaccomplished by firing the resistor ink on an electricallynon-conductive substrate and then taking measurements of its electricalresistance at room temperature such as 73F and at a higher temperaturesuch as 173F and calculating the T.C.R. from these values of thefollowing formula:

7 where A R equals the resistance of the resistor at the aforementionedhigh temperature minus its resistance at the aforementioned roomtemperature.

R is the resistance value at room temperature and A t equals thedifference between the aforementioned high temperature and roomtemperature.

If the T.C.R. value of the resistor is zero, no further modification ofthe ink is needed. If it is not zero audit is negative, then a metalsuch as gold is added to the ink. It is then blended and a measurementof its T.C.R. value is again made in a manner similar to. that alreadydescribed.

The amount of metal added to the ink, such as gold must be large enoughto provide a positive T.C.R. value of not less than parts per millionper degrees centigrade. If the T.C.R. of the ink under modification isfound to be positive then metal oxide, e.g., powder 325 mesh, rutheniumoxide, is added until the ink provides a negative T.C.R. resistormaterial of 20 parts per million per degrees centigrade of a highernegative number. The purpose of the above modification of the availablecommercial inks is to make a pair of inks so that one of the pair willhave a negative T.C.R. resistor value and the other of the same pairwill have positive T.C.R. value. These two inks are then blended bymixing them together in a proportion that will provide a blend of zeroT.C.R. ink.

The amount of positive T.C.R. ink and the negative T.C.R. ink formingthe blended proportion is calculated from the following equation and isdone as explained below:

Where:

P of positive T.C.R. ink in blend for manufacture of zero T.C.R.resistors Exp base of natural log A 1n (100+s) B In S S a constant,based on statistical data derived from experimentation for rutheniumsystem inks which is equal to 3 T T.C.R. of positive T.C.R. ink T T.C.R.of negative T.C.R. ink Both T and T are in PPM/C. S is dimensionless.

The percentage of positive ink in a blend which will provide zero T.C.R.resistors is found through the use of the aforementioned equation. Thissame percentage can also be found graphically by first plotting theT.C.R. of the positive ink on the semi-log paper chart as shown inFIG. 1. The point T plotted on the semilog chart shown in FIG. 1 is theT.C.R. of a positive ink or in other words is the T.C.R. ofa blend whichconsists of 100% positive ink. The abscissaof this point does notcorrespond with the 100% point on the abscissa axis but insteadpurposefully corresponds with the 103% point. This offset of 3% is thevariable S in the aforementioned equation. In this particular examplewhere ruthenium system ink is used it has been found by experimentationthat the value of S 3.

The T.C.R. of a negative T.C.R. ink is then plotted as an ordinate on alinear scale on the semi-log chart of FIG. 1 as the point To. Thisrepresents the value of a blend which has zero percent of positive inkin it. The abscissa, or log scale value, of this point does notcorrespond with the zero percent point on the abscissa axis, but rathercorresponds with the 3% point selected as a result of'statistical dataderived from experimentation. This offset of 3% is variable S in theequation. After the positive and negative T.C.Rs of a pair of inks areplotted as described above the percentage of positive ink P which shouldbe in the blend to provide zero T.C.R. resistors is found as follows:

A straight line is drawn between T and T This line represents a changein T.C.R. of resistors versus percentage of positive T.C.R. ink in theblend and crosses the zero T.C.R. line. Looking at the base of the graphimmediately below the point at which the aforementioned line crosses thezero T.C.R. line we find that its value as read on the abscissa is thepercentage of positive ink in the blend which will provide desired zeroT.C.R. resistor value. It should be noted that the value of this pointalong the abscissa is the value of the P shown in the previouslymentioned equation.

Knowing the percentage P of the positive ink in the blend the zeroT.C.R. blend can then be prepared. However, the T.C.R. of the resistorsmade from this blend will not necessarily be zero; it may not even bewithin the zero plus or minus 20 parts per million per degree centigradelimits. This is so because the previously mentioned equation representsthe best fit or linearized condition that can be derived from the T.C.R.versus log percentage of blend that exists for several differentblending proportions. The degree of misfit depends on the number of testblends and on the ink composition. If the T.C.R. of resistors preparedfrom this blend is not zero as claculated from the previously mentionedequation A R TCR W 10 in PPM/"C or is not within the desired limits, theblend must be corrected. It is evident that the process parametersrelating to the preparation of resistors must be constant. For example,the firing profile, absolute humidity in the furnace, atmosphere in thefurnace, and the dried thickness of the resistors must be kept constant.

The following blending proportion correction is performed in order tobring T.C.R. of the resistor closer to a zero value.

The actual value of the T.C.R. of the resistor, T.C.R. as derived fromthe equations AR 'W" is determined from representative samples of theblend and is plotted in FIG. 1. If this T.C.R. is positive as indicatedby its plotted position in FIG. 1, this point T.C.R. is connected withthe already plotted point T or in other words, the point which is theT.C.R. value of the negative T.C.R. ink. This line between the points'T.C.R. and T represents a corrected change in T.C.R. of resistors versuspercentage of positive T.C.R. ink in the blend or, in other words, thechange T.C.R. of resistors versus percentage of positive T.C.R. ink inthe blend which was previously determined in FIG. 1 was incorrect due toimperfect linearization when parame- 9 ters were chosen as previouslydescribed for the first previously mentioned equation that was used tofigure out the value of P.

If this TCR, were negative, e.g., TCR, this TCR, point would beconnected by a straight line to the point T The line connecting pointTCR, with point T or TCR, with point T must in each instance cross thezero T.C.R. line. In one example, the T.C.R. of the resistors made froma blend prepared by the previously mentioned graphical method ispositive and plotted at its point TCR, in FIG. 1. The abscissa of thepoint of intersection or point I, on FIG. 1 between the TCR -T line andthe zero T.C.R. line is a corrected percentage of positive ink in theblend which shall provide zero T.C.R. resistors and is marked on FIG. 1as P Knowing F which is the corrected and more accurate percentage ofpositive T.C.R. ink in the blend, the blend can be either corrected byadding corresponding amounts of negative T.C.R. ink to the blend or anew second blend can be prepared based the information derived in theaformentioned manner.

Even now, the second corrected blend may still not provide zero T.C.R.resistors. If this is the case, a second correction is needed and theT.C.R. of the resistors made from No. 2 blend as determined from theequation AR 8 TCR 10 in FIG. 1 as point TCR is then determined. In thisexample, TCR, turned out to be negative. This is accomplished by drawinga line through this point TCR and the previously obtained point TCR Thisline represents the second corrected change of T.C.R. versus percentageof positive ink in the blend. The abscissa of the point of intersectionbetween the line TCR TCR and the zero T.C.R. line is a correctedpercentage of positive inks in the blend which will provide zero T.C.R.resistors and is marked P in FIG. 1.

Knowing P which is the second corrected percentage of positive T.C.R.ink in the blend, this blend can then be either corrected by addingcorresponding amounts of positive T.C.R. ink, for example, ink with TCRT or a new third blend can be prepared based on the aforementionedinformation.

In the above example, the T.C.R. of the second blend TCR was negative.If it were positive then the TCR point would be connected by a straightline with point T and the abscissa of the point of intersection betweenline TCR -T and the zero T.C.R. line would be the percentage of positiveT.C.R. ink in the third blend. When an additional correction of theblend is needed, such as in the case where the T.C.R. of the resistorsmade from the blend are outside of the desired limits, the last obtainedand plotted T.C.R. point, e.g., TCR is then connected with the nearestT.C.R. point of opposite sign as measured along the abscissa. Theabscissa of the intersection point of this last mentioned line whichconnects the two nearest T.C.R. points of opposite signs with the zeroT.C.R. line represents the percentage P; of positive T.C.R. ink whichshould be in the corrected blend. 1

Experimentation has shown that in the majority of blending operationsonly two such corrections are sufficient to bring the T.C.R. within thei parts per million per degree C limits.

It should also be further understood that a method has been describedthat can be used for obtaining any desired T.C.R. for resistors otherthan zero by observing where the interconnecting line between T and Tpasses a horizontal line on the chart that passes through the desiredpositive or negative value of the blend that is desired rather thanthrough the zero T.C.R. line. This T.C.R. of the blend cannot, ofcourse, be made more negative or more positive than the T.C.R. value ofthe two basic inks that were used to make this blend.

The change in T.C.R. resistors causes the change in the sheetresistivity of the resistors and the more negative that the T.C.R. isthe higher will be the sheet resistivity. This is so because theaddition of metallic oxide to the ink causes the T.C.R. to change in thenegative direction and increases the sheet resistivity.

Knowing the sheet resistivity of the two inks which are used forblending and knowing their percentage in the final blend the sheetresistivity of resistors made from this blend can be easily predicted byusing known methods of calculation.

METHODS OF TRIMMING OF HIGH ACCURACY RESISTORS (CERMET) WITH LOWACCURACY TRIMMING MACHINE Present day accuracy of cermet resistors afterthey are printed and fired is about i 20% of the value desired.Therefore, if a better accuracy is desired, they must be corrected.Usually, the correction consists of removing a portion of the resistoruntil the resistance reaches the desired value. A partial removal ofresistor material causes an increase in the resistance. In other words,the value of resistance can be corrected only in the direction ofincrease of the resistance.

Usually the resistance of the resistor under trimming is constantlymeasured by using a high precision resistance measuring bridge, forexample, a Kelvin bridge.

The accuracy of resistance measuring bridges is usually of i 0.05%.However, the accuracy of trimmed resistors is seldom better than i: 1%.This is caused by the unpredictable time lag between the electricalsignal from the bridge, indicating that the resistor has reached itsdesired value and the execution of the signal (i.e., stopping thetrimming) by conventional electromechanical and pneumatic links betweenthe bridge and the cutting device. The degree of overtrim or, in'otherwords, overcuts, depends on the speed of the cutting device which isusually a nozzle which directs the stream of abrasive particles on theresistor and also on the resistivity of the resistors material. Thehigher the nozzle speed and the resistivity, the larger will be overtrimor error. Usually, the degree of overtrim does not exceed 1% of nominaldesired resistance. In other words, even if the trimming machine, whichincludes resistance measuring bridge and the cutting devices, has a highprecision bridge, its total accuracy, i.e., the accuracy of trim,usually is in a low precision range.

Described below are two methods of trimming a high precision resistor 10with low precision trimming ma chines, which have a high precisionresistance measuring bridge. The first method is as follows:

The resistor 10 is laid out so that it consists-of two parts 12 and 14as shown in FIG. 2. Part 12 measured between points a and b ofconductive parts l6, 18 must be of sufficient size and length to provideat least 98% of the nominal desired total resistance after trimmingalong trimming path 20. Part 14 resistor measured between the poirits band c of conductors 18, 22 must have not more than 1% of the nominaltotal resistance "before it is trimmed along the trimming path 24.

Measuring across the entire resistor, i.e., between the points a and cof conductors 16, 22, the resistor part 12 is trimmed to 98% of thetotal nominal resistance because the accuracy of trimming machine is i1%. The resistance of the entire resistor measured between a and c willbe 98% i 1% of the total nominal resistance and the resistance of justtrimmed resistor alone measured between a and b of conductors l6 and 22can be 98% of nominal i 1% of nominal R where R is the resistance of theuntrimmed part 14 measured between 18 and 22.

As it was mentioned before, the maximum resistance of untrimmed Rresistor 14 does not exceed 1% of total nominal resistance, therefore,in the worst case, the minimum resistance of just trimmed R resistor is98% 1% 97% of the total nominal resistance.

To correct the error after the first trim along trim path 20 theactualvalue of the trimmed R resistor part 12 must be measured. Sincethe resistance measuring bridge only is involved in this measurement,the measured value of R resistor 12 will be within the accuracy of thebridge, i.e., usually within i 0.05%. This uncertainty in the trimmedR,,,, value can obviously not be corrected, and it depends on theaccuracy of mea suringbridge alone.

Assumingthat R resistance of resistor part 12 after trim was 97% oftotal nominal resistance, the untrimmed resistor 14 must be trimmedalong trimming path 24 until it reaches 100% 97% 3% of the total nominalvalue. I

Because of i 1% accuracy of the trimming machine, the resulting value oftrimmed resistor 14, measured between points b and during the trimming,and trimmed along trim path 24 will be 3% of total nominal resistance i1% of 3% of nominal or 3% i 0.03% of total nominal value versus desired3% of nominal. Assuming one of the worst possible cases, the resistanceof trimmed resistor part 14 can be 3% 0.03% 2.97% of the total nominalvalue; and the total value will be R (part 12) R,,, (part 14) 97% 2.97%99.97% of the total nominal value, i.e., the error after two trims willbe 0.03%. Adding the uncertainty or the resistance measuring bridge0.05%) the maximum error after two trims will be (in this example) i0.05 0.02 0.08%.

The above example shows that, dividing the resistor into two parts l2,l4 and performing two trims 20, 24, the final accuracy obtained is morethan ten times better thann the accuracy of conventional trimmingmachine and that it approached the accuracy of a precision resistancemeasuring bridge.

The accuracy obtainable by this two trim method is determined asfollows:

(R1 RI R2) Total maximum error in R after trimming (in of nominal) Afterfirst trim described above; R =(1A)R i (lA) RA NOTE: The resistor istrimmed to l-A) of nominal value. A is expressed in decimals.

R trimmed R R untrimmed (lA) R (lA) RA BR R [1(A+B): :A(1--A)] Note: Bis expressed in decimals Error in R, trimmed R R (A+B) i A (lA -R (A+B)i A (1A)- L CR Note: C is expressed in decimals. The error in R trimmedis measured with res. bridge of :t C accuracy.

A-fter the second trim, i.e., after R, is trimmed to [R trimmed R]value:

R ftrimmed R [(A+B) 1A (lA)] CR i AR [(A+B)$A (1-A)]=R {(A+B)-TA(lA)i-CR i A [(A+B) IA (lA) 1} =R {(A+B) IA (lA) C: [A AB i-A (lA)]} R Rtrimmed +.R trimmed R {[1 (A+B) A (lA)] [(A+B) IA (lA)] :C-J:[A-l-AB:A,(,1-A)]} =R {1 (A+B) :A (lA) +(A+B) +4 (.l-A) C: [A AB 1- A (1-A)])Error in R =R -R=R {-(A+B):A (l-A) +(A+B) I A (lA) i c: [A AB tA (1A)lR1; Ci: [A +ABi-A (lA)]} '-R {A +AB:

A i A i C} Max error in R =:R {2A +AB +A C} max error in R max error inR Note: A, B, C, expressed in A numerical example where A i 1%, B 1%, C.05%, as it was described before will yield the following accuracy inthe resistor trimmed by described method:

Max. error i (2 (0.01%) 0.01% 0.0001% 0.05%) 1 0 0801%. A second methodof trimming Without changing the resistor position in the trimmingmachine, the resistor is trimmed again to 99.5% of the desired valuealong trimming path 28. In other words, the cutting device (whichusually is a nozzle which provides a jet of abrasive particles suspendedin air) repeats the same cutting pattern. Experiments have shown thatthe amount of resistors material removed by this second trimming isabout 0.5% of that removed in the first trimming. This is equivalent toslowing down the trimming speed by the factor of 1/200.

The same trimming pattern is repeated for a third time along trimmingpath 30 and the resistor is trimmed to its desired value. The amount ofresistor material removed by this third trim is about 0.05% of thatremoved by the first trim, which is equivalent to slowing down thetrimming speed by the factor of 1/2000 as compared with the firsttrimming period.

The accuracy of the trimmed resistors (assuming the accuracy ofresistance measuring bridge as i0.05%) is usually in the order ofODS-0.09%, which is comparable with the first described method.

The accuracy of trimmed resistors can be improved further if four trimsare used instead of three, approaching the accuracy of the resistancemeasuring bridge.

Both of the methods described herein allow a single thick film resistorto be trimmed to any one of a number of desired values.

The aforementioned precise trimming method enables a reduction to bemade in the cost of manufacturing resistors having different resistorvalues because the same common blend of positive and negative resistorink having a zero T.C.R. can be fired onto each one of a number ofsubstrates before different individual selective trimming of each ofthese resistors occurs.

It should be noted that trimming of the cermet resistor by either of theaforementioned methods is done after the previously described selectedzero T.C.R. blend of positive and negative cermet resistor ink that wasused to form resistor 10 has been fired onto the aluminum oxidesubstrate 32.

When a thick film cermet resistor 10 of the aforementioned type is leftexposed to its surrounding atmosphere its precisely manufacturedresistance and T.C.R. value will be altered with time because of thedestructive oxidation and other similar detrimental effects which air,moisture, hydrogen sulfide or other similar destructive delequescentmaterials have on the resistor 10.

More particularly, the stability of cermet resistors that are notprotected from the ambient atmosphere, whether under a load or no loadcondition, is usually in the order of 0.3-0.5% per year. In other words,the resistance of these resistors have heretofore changed by O.30.5% ayear after they are manufactured.

Such a poor stability precludes the possibility of the manufacture ofhigh precision resistors which have a tolerance of i 0.1% or better.

Manufacturing a thick film resistor in the manner to be hereinafterdescribed provides a resistor which will retain a stability of 0.1% forat least 2 years.

In other words, the resistance of these resistors will change no morethan 0.1% of their nominal value after two years of active use in acircuit or during the period in which they are stored on the shelf forthis length of time.

Experimental tests showed that the main reason for the instability ofthick film resistors or, in other words, drift in resistivity with timewas caused by oxidation of the metals in the resistor and by absorptionof water, contained in the atmosphere. Therefore, the resistors must beinsulated from the ambient atmosphere.

To solve this problem, an insulator layer must be provided which has thesame temperature coefficient of expansion as the ceramic substrate 32and the resistor 10 and conductor 16 and 22. Otherwise, thermal stresseswill develop with an accompanying change in resistance. Another way isto make the insulating layer flexible enough to prevent stresses fromoccurring in the resistor which would change its resistance by more thani 0.05% for the desired specified temperature range, e.g., a change inambient temperature. Also, the layer which physically contacts theresistor must be chemically inert with respect to resistor material, forexample, it should notcause oxidation or reduction of the resistormaterial to occur and at the same time it must be able to adhere to theresistor 10. Another factor that had to be considered was that since theflexible insulation layer must possess a soft flexible characteristic itneeds additional protection from mechanical damage such as scratches,etc. The encapsulating structure on the substrate as described belowprovides a system of layers to protect the cermet resistor from ambientair, water vapors, water, and hydrogen sulfide (I-I S). Furthermore, thelayers to be described have been found satisfactory in protectingtheresistor from being affected when continuous changes in ambienttemperature that may vary from the standard reference level of 25C 1 50Cso that no more than i 0.1% change in value of the resistor can occur.

The ceramic substrate 32 which is preferably at least 96% pure aluminumoxide material is exposed to 1000C for 15 to 20 minutes. It is assumedthat the substrate 32 is clean prior to this operation; if it is not, itis cleaned ultrasonically in ethyl or methyl alcohol for 3 minutes.Next, the previously mentioned resistor 10 and conductor inks 16, 22 areprinted, dried and fired in the manner previously described as shown inFIG. 6 of the drawing.

A silicone polymer filled with magnesium oxide 34 such asdimethylpolyxilaxane which is commercially available from the EMCACompany as plastic coat 1139B is then printed or brushed over theresistor 10 and conductor areas 16, 22 except for the conductor areasthat are reserved for the terminals 38 and 40. The substrate 32 is thenheat cured at 108C for 24 hours to provide polymerization and theresistor 10 is trimmed through the plastic coat 34 to 99 i 1% of itsdesired value as previously described and the terminals 38, 40 are thensoldered with suitable soldering material 42, 44 as shown in FIG. 6.

Next, the substrate 32 is heat cycled twice between 25C to C at thetemperature-time slope of 20C per minute and kept for 2 hours at 125Cthen cooled down on the same rate to 25C. The same heat cycle isrepeated for a second time, and then for a'third time for a period of 15hours instead of 2 hours and at the same temperatures.

.The resistor is then finally trimmed as previously described under thedescription of FIGS. 2-5 to minus 0.03% of the desired value. Theresistor is then cleaned with a jet spray of nitrogen. A mixture of ironoxide with magnesium silicate suspended in xylene 36 such as glyptal1201B paint that is commercially available is sprayed over the entiresubstrate including the resistor conductors and portions which form thesolder joint and terminals. The substrate is then exposed to 100C for 4hours.

The thickness of the flexible silicone polymer layer 34 that is selectedis never less than 12 microns and the thickness of the hard glyptallayer 36 is not less than 50 microns. A cross-sectional view of theprojected resistor 10 is as shown in FIG. 6.

Experimentation has also shown that cermet resistors that are preparedin the above-described manner will remain stable within 0.1% for atleast two years or more.

The plotted dotted line shown in FIG. 7 indicates that it is possiblethrough the use of a conventional diffusion introducing profile firingmethod to obtain only a single resistor blend of ink that has a zeroT.C.R. value from a series of different blends of inks which possessdifferent sheet resistivity values.

FIG. 7 also shows a second plotted solid line to indicate that a seriesof resistors having different resistivity values over a wide resistivityrange can be obtained which each has a zero T.C.R. value when theresistor is first fired by the unique non-diffusing method previ ouslydescribed in which the resistor is first fired to the substrate at onetemperature and the conductor and resistor are thereafter jointly firedat a second tempera ture that is approximately 500C lower than the firstmentioned temperature.

The unique steps employed in the preparation of a zero T.C.R. thick filmresistor are:

l. Ultrasonically clean substrate 32 in methanol for 30 seconds.

2. Prefire substrate 32 at lOC.

3. Clean substrate 32 with N print resistor.

4. Dry resistor at 107C for 45 minutes.

5. Ascertain the correct firing temperature of the furnace.

6. Fire resistor at a plateau temperature of 1000C on a 2-inch perminute moving belt.

7. Clean substrate 32 with N and print conductors 8. Dry conductors 16,22 at 107C for 45 minutes.

9. Ascertain the correct firing temperature of the furnace.

10. Fire the conductors 16, 22 at a plateau temperature of 550C on atwo-inch per minute moving belt.

11. Anneal by heat cycling at 177C for 15 hours.

12. Clean resistor 10 and conductors 16, 22 with N and screen onflexible layer 34.

13. Dry flexible layer 34 at 126C for 12 hours.

14. Stake pins 38 and 40 and solder at 42, 44.

15. Trim resistor 10-98% of its normal resistance value and clean with N16. Heat cycle at 121C two times for 2 hours and then overnight toeliminate stresses induced by trimming.

l7. Trim to desired value and clean with N 18. Spray on hard layer 36.

19. Dry hard layer 36 at 93C for 4 hours.

The significance of eliminating the harmful effects the diffusion has onT.C.R. and resistivity which has heretofore been brought about by firingthe resistor and conductor at substantially the same high temperature isclearly illustrated in FIGS. 8 and 9.

FIGS. 8 and 9 show, for example, how T.C.R. and the resistivity of thickfilm resistors are dependent on the previously referred to geometry, oraspect ratio of the resistor when they are'fired with associatedconductors at the same high temperature and how this dependence waspractically eliminated when the unique process heretofore described wasemployed.

Curve A in FIG. 8 shows the dependence of T.C.R. on the aspect ratiowhen a thick film precision resistor is manufactured by usingconventional methods in which the resistor and conductor is fired at thesame high temperature. It can be seen in this conventional method thatthe T.C.R. value changed from +74 PPM/C at the aspect ratio of 0.1 to-56 PPM/C at the aspect ratio of 10. In other words, curve A shows thata total change in T.C.R. of PPM/C occurred when the previously mentionedruthenium system ink is used as the resistor material, platinum gold inkis used as the conductor material and after the resistor ink andconductor were fired at the high temperature of lO00C.

FIG. 8 curve B shows how the dependence of T.C.R. on the aspect ratio isfor all practical purposes eliminated when the thick film resistor ismanufactured by the previously described unique method of manufacturing.

The dependence of T.C.R. on the aspect ratio decreases from 130 PPMICfor conventional methods that have heretofore been used as shown in FIG.8 curve A to 28 PPM/"C for the unique method of manufacturing that hasfor the first time been disclosed herein.

F urthennore, in addition to the decrease in the T.C.R. dependenceon'the aspect ratio it can be seen that the T.C.R. versus aspect ratiocurve shifts in a negative direction after the unique manufacturingmethod was used.

More specifically, both curves A and B represented the relationshipbetween the T.C.R. and the aspect ratio for the same ruthenium systemink resistor material. The only difference in the manufacturing processdepicted in the curves A and B shown in FIG. 8 is that in curve A theresistors and their associated conductors were fired at approximatelythe same temperature such as about I000C and for curve B the resistorswere first fired at I000C and thereafter the resistors and theirassociated conductors were jointly fired at about 550C.

It should also be noted that the conductor which is connected to theresistor represented by curve B contains silver combined with glassinstead of the conventional platinum-gold (PtAu) combined with glasstype conductor as represented by curve A. The only reason for the changein conductor material from PtAu to silver was that it was not possibleto fire a PtAu type conductor at 550C whereas a silver type conductorcan be fired at this temperature.

A shift of curve B in a negative direction shows that the degree ofdifi'usion of conductor material into the resistor material hasdecreased and that the true T.C.R.

of the resistor ink is in reality that shown on curve B rather than thegenerally heretofore assumed value that is shown on curve A.

Curve C, FIG. 8, depicts the dependence of T.C.R.

I on the aspect ratio after the resistive ink was blended 17 and 8 PPM/CT.C.R. and its dependence on the geometry (aspect ratio) for allpractical purposes is nil.

FIG. 9, curve A shows the dependence of resistivity on the aspect ratiowhen the thick film resistors are manufactured by conventional methods.

2l50 ohm per square per mil was the change in resistivity that occurredwhen a change in aspect ratio went from 0.1 to 10.

Curve B of FIG. 9 provides a way of showing the independence ofresistivity on the aspect ratio when the resistor is manufactured by theunique method previously described for FIG. 8, curve B.

The dependence of resistivity on the aspect ratio (geometry) decreasesfrom 2150 ohm per square per mil on curve A to 1 100 ohm per square permil on curve B.

FIG. 9, curve C shows the resistivity versus aspect ratio after blendingas previously described under FIG. 8.

It has been determined by experimentation that substantially 20% byweight of RuO 40% by weight of Ru and 40% by weight of glass frit is onetype of positive temperature coefficient of resistivity resistor inkthat can be employed to advantage in the aforementioned described inkmixtures that are formed from positive and negative temperaturecoefficient of resistors ink. It has also been determined by experimentthat substantially 40% by weight of RuO 20% by weight of Ru and 40% byweight of glass frit is one type of negative temperature coefficientresistivity resistor ink that can be advantageously used in theaforementioned ink mixture to make the resistor 10.

It can, therefore, be seen that the unique apparatus and method of firstfiring the resistor onto a substrate 32 at lO00C and the later jointfiring of the resistor l0 and its associated conductors 16, 22 onto thesubstrate 32 at a lower temperature, namely 550C, will substantiallyeliminate diffusion that has heretofore occurred between the conductormaterial and the resistor material.

By procuring a resistor and its associated conductors after firing inthe substantially same undiffused state that they were in before firing,it is for the first time possible to eliminate the T.C.R. andresistivity dependency on the geometry of the resistor commonly referredto as aspect ratio that has heretofore existed when other firing meanshave been employed for this purpose.

Because of the aforementioned advantages derived from the unique firingtechnique, it is now possible for the first time to manufactureprecision thick film resistors without concerning oneself with theheretofore existing problem of:

1. Selecting the right aspect ratio or. in other words, the length-widthratio, or the geometry, of the resistor that is to be fired onto asubstrate,

2. Spending time in consulting geometry correcting tables to predictresistivity and the T.C.R. of the resistor as a function of theresistors geometry, and

3. Requiring the creative ability of the designer who is designing anelectrical, thick film circuit from being able to present the mostdesired economical compact circuit because the resistors that haveheretofore been used were required to be of a prescribed geometric shapeand size.

The embodiments of the invention in which an exclusive property orprivilege is claimed is defined as follows:

1. A method of making an encapsulated thick film resistor havingconductive portions associated with opposite ends thereof that formportions of an electrically conductive circuit, comprising the firststep of selecting an ink from a mixture of positive and negativeresistor inks, the second step of firing the resistor ink mix inamorphic form at 1000C into a sintered state onto an electricallynon-conductive substrate, the third step of selecting a conductive inkmaterial that has a sintering temperature substantially lower than thesintering temperature of the resistor, the fourth step of firing saidconductive ink at a sintering temperature of about 550C in overlappingrelationship with said opposite ends of said resistor on the substratewhereby the tendency to diffusion between the conductive and resistivematerials is minimized and whereby a resulting temperature coefficientof resistivity value of the resistor that is produced is within plus orminus 20 parts per million per degree Centigrade from zero, the fifthstep of applying a coating of resilient buffer material formed of asilicone polymer filled with magnesium oxide to cover said resistor andthe said fired conductive ink and the sixth step of applying a layer ofiron oxide with magnesium silicate suspended in xylene to cover theouter surface of said resilient material.

1. A METHOD OF MAKING AN ENCAPSULATED THICK FILM RESISTOR HAVINGCONDUCTIVE PORTIONS ASSOCIATED WITH OPPOSITE ENDS THEREOF THAT FORMPORTIONS OF AN ELECTRICALLY CONDUCTIVE CIRCUIT, COMPRISING THE FIRSTSTEP OF SELECTING AN INK FROM A MIXTURE OF POSITIVE AND NEGATIVERESISTOR INKS, THE SECOND STEP OF FIRING THE RESISTOR INK MIX INAMORPHIC FORM AT 1000*C INTO A SINTERED STATE ONTO AN ELECTRICALLYNON-CONDUCTIVE SUBSTRATE, THE THIRD STEP OF SELECTING A CONDUCTIVE INKMATERIAL THAT HAS A SINTERIG TEMPERATURE SUBSTANTIALLY LOWER THAN THESINTERING TEMPERATURE OF THE RESISTOR, THE FOURTH STEP OF FIRING SAIDCONDUCTIVE INK AT A SINTERING TEMPERATURE OF ABOUT 550*C IN OVERLAPPINGRELATIONSHIP WITH SAID OPPOSITE ENDS OF SAID RESISTOR ON THE SUBSTRATEWHEREBY THE TENDENCY TO DIFFUSION BETWEEN THE CONDUCTIVE AND RESISTIVEMATERIALS IS MINIMIZED AND WHEREBY A RESULTING TEMPERATURE COEFFICIENTOF RESISTIVITY VALUE OF THE REISTOR THAT IS PRODUCED IS WITHIN PLUS ORMINUS 20 PARTS PER MILLION PER DEGREE CENTIGRADE FROM ZERO, THE FIFTHSTEP OF APPLYING A COATING OF RESILIENT BUFFER MATERIAL FORMED OF ASILICONE POLYMER FILLED WITH MAGNESIUM OXIDE TO COVER SAID RESISTOR ANDTHE SAID FIRED CONDUCTIVE INK AND THE SIXTH STEP OF APPLYING A LAYER OFIRON OXIDE WITH MAGNESIUM SILICATE SUSPENDED IN XYLENE TO COVER THEOUTER SURFACE OF SAID RESILIENT MATERIAL.